The present invention relates to telecommunications networks, and more particularly, to pure optical switches which direct light pulses from one optical fiber to another without electrical conversion.
Telecommunications service providers continue to seek ever greater bandwidth at ever lower prices. Their data networks must be flexible to allow for continual upgrades, also referred to as xe2x80x9cprovisioningxe2x80x9d. They must also designed for rapid fault recovery to avoid service degradation and even outages. High speed optical data networks now carry most of the long haul, and much of the metropolitan area data traffic in developed countries. Along such networks microprocessor controlled routers perform so-called xe2x80x9cOEOxe2x80x9d transcriptions, converting optically encoded data received from input optical fibers to electrical signals, reading destination code, and then reconverting the electrical signals back to optically encoded data and sending it along output optical fibers. As transmission speeds pass 2.488 Gbits/sec (OC-48 level), this conversion step becomes more difficult to perform and the cost of conventional high throughput electrical switches becomes unacceptable.
Pure optical switches direct light pulses directly from one optical fiber to another without electrical conversion and therefore offer the promise of eliminating much of the OEO transcriptions in high bandwidth fiber optic data transmission networks. Electrical routing intelligence would still be needed to direct traffic. However, currently about eighty percent of the traffic handled by a conventional router passes straight through and reading the destination header in most cases is a waste of time and system resources. By separating the control information from the transmitted data, pure optical switching would bring substantial increases in the throughput rate of optical data networks.
A variety of miniature electromechanical devices have been developed for changing the path of light in free space to direct light pulses from one optical fiber to another optical fiber. One promising approach utilizes three dimensional (3D) microelectromechanical systems (MEMS). Generally speaking, MEMS fabrication technology involves shaping a multi-layer monolithic structure by sequentially depositing and configuring layers of a multi-layer wafer. The wafer typically includes a plurality of polysilicon layers that are separated by layers of silicon dioxide and silicon nitride. The shaping of individual layers is done by etching that is controlled by masks patterned by photolithographic techniques. MEMS fabrication technology also entails etching intermediate sacrificial layers of the wafer to release overlying layers for use as thin elements that can be easily deformed and moved. Further details of MEMS fabrication technology may be found in a paper entitled xe2x80x9cMEMS The Word for Optical Beam Manipulationxe2x80x9d published in Circuits and Devices, July 1997, pp. 11-18. See also xe2x80x9cMultiuser MEMS Processes (MUMPS) Introduction and Design Rulesxe2x80x9d Rev. 4, Jul. 15, 1996 MCNC Mems Technology Applications Center, Research Triangle Park, North Carolina 27709 by D. Keoster, R. Majedevan, A. Shishkoff and K. Marcus.
FIG. 1 is a diagrammatic illustration of a conventional 3D MEMS optical switch 10. A first array 12 of micro-machined mirrors is aligned with an input optical fiber bundle 14, and juxtaposed opposite a second array 16 of micro-machined mirrors. Electrical command signals from a switch controller (not illustrated) cause individual mirrors in the arrays 12 and 16 to tilt. Input light pulses transmitted through a selected fiber in the input bundle 14 that strike an individual mirror in the first array 12 can be directed to another specific mirror in the second array 16 and from that mirror to a selected fiber in an output optical fiber bundle 18 aligned with the second array 16. The individual light beams travel along Z-shaped paths 19 in free space. There is usually a lens (not illustrated) between the first and second mirror arrays 12 and 14. The purpose of this lens is to image the facets of the fibers in the input bundle 14 onto the facets of the fibers in the output bundle 18. Because the light beams coming out of the fibers in the input bundle 14 diverge, the lens is necessary to focus the light onto the fibers in the output bundle 18. In some cases, there are two lenses between the two arrays 12 and 14 to form a sort of telescope in order to accomplish this imaging. The optical switch 10 has distinct advantages over electrical switches in that the former operates completely independent of changes in the bit rate, wavelength and polarization.
3D MEMS optical switches are targeted for use in network cores and nodes in both long haul and metropolitan area data networks. 2D MEMS optical switches simply raise or lower pop-up mirrors at fixed angles to switch to a given data port. See for example U.S. Pat. No. 5,994,159 of Aksyuk et al. assigned to Lucent Technologies, Inc. and U.S. Pat. No. 6,097,859 of Sogarard et al. assigned to the Regents of the University of California. In the 3D MEMS optical switch of FIG. 1, optical signals are reflected by the first and second arrays 12 and 16 each made of micro-machined mirrors that can each be tilted variable amounts in two axes, bouncing an incoming optical signal from a selected optical fiber in the input bundle 14 to a selected optical fiber in the output bundle 18 in a manner that results in less signal loss than in 2D MEMS optical switches.
The 3D MEMS optical switch of FIG. 1 accommodates any data rate or transmission protocol and its architecture is more readily scalable than 2D MEMS optical switch designs. Larger switching capacities are achieved simply by doubling, rather than squaring, the number of mirrors needed for the desired channel count. 2D MEMS optical switches are really not practical beyond a 32xc3x9732 matrix. 3D MEMS optical switches have been commercially announced that offer a 64xc3x9764 input/output capacity in a relatively small form factor. They have high cross-talk rejection and flat passband response and are well suited for use in wavelength-division multiplexed (WDM) optical data networks.
While 3D MEMS optical switches show great promise, precise angular alignment of the miniature mirrors can be difficult to achieve. Precise alignment is needed in order to minimize optical losses.
It is therefore the primary object of the present invention to provide a system and method for actively aligning titling mirrors in a pure optical switch.
In accordance with a first embodiment of our invention a system for actively aligning mirrors in an optical switch includes a plurality of input optical fibers, a plurality of output optical fibers and at least one array of tilting mirrors. Each tilting mirror receives a beam of light from a corresponding one of the input optical fibers and directs the beam of light for receipt by a predetermined one of the output optical fibers. A plurality of actuators each progressively tilt a corresponding one of the mirrors. A plurality of fractional taps such as directional optical couplers, dichroic mirrors, optical wavelength multiplexer/de-multiplexer devices or other devices each redirect a portion of the beam of light received by a corresponding one of the output optical fibers. A plurality of detectors are each optically coupled to a corresponding one of the optical taps. A control circuit is connected to the detectors and to the actuators and precisely tilts each mirror to minimize losses in optical signal power resulting from the routing of each light beam as it is switched from a predetermined one of the input optical fibers to a predetermined one of the output optical fibers.
In accordance with a second embodiment of our invention a system for actively aligning mirrors in an optical switch includes a plurality of input optical fibers, a plurality of output optical fibers, a first array of tilting mirrors and a second array of titling mirrors. Each mirror in the first array receives an information beam of light from a corresponding one of the input optical fibers and each mirror in the second array receives the information beam from a corresponding mirror in the first array and directs the information beam for receipt by a predetermined one of the output optical fibers. A first plurality of actuators each progressively tilt a corresponding one of the mirrors of the first array. A second plurality of actuators each progressively tilt a corresponding one of the mirrors of the second array. A first plurality of mechanisms such as directional optical couplers each connect an emitter to a corresponding one of the output optical fibers for injecting an alignment beam of light. A second plurality of mechanisms such as directional optical couplers each connect a corresponding one of the input optical fibers for receiving the alignment beam transmitted therethrough. A plurality of detectors are each connected to a corresponding one of the second plurality of directional optical couplers and each generate an electrical signal representative of the alignment beam. A control circuit is connected to the detectors and to the actuators. The control circuit precisely tilts each mirror to minimize losses in optical signal power resulting from the routing of each light beam switched from a predetermined one of the input optical fibers to a predetermined one of the output optical fibers.
In accordance with a first embodiment of our method for actively aligning mirrors in an optical switch, a plurality of information light beams are transmitted through free space between corresponding optical fibers in an input bundle and an output bundle utilizing a plurality of arrays of tilting mirrors to direct the information light beams. Each corresponding optical input fiber and optical output fiber define a channel. A loss in optical power in each of the channels is detected by tapping into each optical output fiber. An alignment signal is generated for each channel based on the loss detected for that channel. The alignment signal is utilized to control an actuator associated with each tilting mirror to direct the information light beam for each channel so as to minimize the optical loss in that channel.
In accordance with a second embodiment of our method for actively aligning mirrors in an optical switch, a plurality of information light beams are transmitted through free space between corresponding optical fibers in an input bundle and an output bundle utilizing a plurality of arrays of tilting mirrors to direct the information light beams. Each corresponding optical input fiber and optical output fiber define a channel. A plurality of alignment light beams are transmitted through free space between corresponding optical fibers in the output bundle and the input bundle utilizing the plurality of arrays of tilting mirrors to direct the alignment light beams. A loss in optical power of the alignment light beam in each of the channels is detected. An alignment signal is generated for each channel based on the loss in optical power of the light beam detected for that channel. The alignment signal is utilized to control an actuator associated with each tilting mirror directing the information light beam for each channel so as to minimize the optical loss in that channel.
The first and second embodiments may also be simplified to provide 1xc3x97N optical switches.